Q. We have an older bittersweet that badly needs to be trimmed back. When should I do this?
Betty Jelinek, Jackson
A. Early spring is the best time to prune bittersweet in Michigan, Betty, and your first order of priority should be to get the oldest branches pruned away. Doing so will improve fruiting.
Bittersweet is a very rapid grower so if your plant hasn't been pruned for some time, you'll likely find a tangle of vines that at first blush seems impenetrable. But it's not quite the tangled mess it seems. Instead of reducing the mass by pruning off tips of all the branches, remove entire branches, especially the oldest ones. Take them off right back to the main stem, or soil. The oldest branches will be the thickest of the bunch and will likely be the ones with the fewest leaves. Take these off first and see what's left. A rough pruning guideline is to take off no more than 20 percent of a plant's bulk at any one time and that usually means no more than 20 percent each year. Ideally, you are pruning on an annual basis to keep the plant's natural shape intact and its size within the space allocated for it. With annual pruning, you'll typically remove 5 to 10 percent.
A very over-grown bittersweet is an exception to the 20 percent guideline. Betty, if your plant is very large and tangled, then I'd prune back one third of its bulk early next spring. That should get it back within bounds and only light to moderate touch-up pruning will be needed in subsequent years.
Two other factors may explain why Betty's bittersweet hasn't set fruit.
One is sunlight. Bittersweet sets the most fruit when it gets full sun all day. Over time, low-growing plants like the bittersweet are increasingly shaded as nearby trees and shrubs grow taller and broader. So what was once a free-fruiting shrub becomes less so with each passing year. This is one of those things around the yard hat seem to take several years before it sinks in that something is not right.
The other explanation for no berries is the plant is lack of a male plant nearby. I wasn't sure about this angle, so I checked my references and learned the bittersweet is usually dioecious. A dioecious plant is one in which male and female flowers are borne on separate plants. Holly is a prime example of this. Holly berries are borne on female plants but to make that happen, there must be a male nearby. With bittersweet, it is not as clear-cut. My references say the plant is usually dioecious. So, I guess that means it may need a male nearby, or it might not.
To have a bittersweet but not to have its fruits is a shame, given the bright and welcome color berries provide through the autumn and winter. In my opinion, it's worth taking steps to correct the problem so the plant will bear fruit. Let's hope that in Betty's case, it's as easy as pruning out the dead wood. Providing more light usually means moving the plant (which can be difficult) or pruning off limbs and branches from nearby trees and shrubs, and that may be too drastic a measure to take just to get the bush to fruit. Adding another plant - a male - is easily done but requires having room for it and a nursery that offers plants of both genders.